1. Field of the Invention
The present invention relates to semi-conductor devices of the field effect transistor type, and more especially those whose cut-off frequencies are measured in gigahertz.
2. Description of the Prior Art
Known transistor type devices work at frequencies up to a few gigahertz. To increase their cut-off frequencies several possibilities are open: change the structure and the dimensions of the device or change the nature of the materials used for making it.
Among the known structures of field effect transistors, currently designated by the abbreviation FET, are: the MOS-FETs, the MES-FETs, the junction FETs, and heterojunction gate FETs of type P. The MOS-FETs has a structure having a metal-oxide-semiconductor material stack, from whence the name MOS, which operate with silicon under type inversion conditions, in the vicinity of the oxide layer. By reducing to a minimum the dimensions of the electrodes and the distances between electrodes so as to reduce the transfer time of a charge carrier, cut-off frequencies of the order of 5 gigahertz can be obtained. The GaAs based MOS-FETs exist in the design stage n the laboratory and operate in the depletion region. Their performances are similar to those of the MES-FETs which will be described hereafter.
The MES-FETs, the simplest structure which only has a metal and a semiconductor (ME-S) operate in depletion, i.e. the charge carriers are deflected, under the control electrode, and pinched, due to this field of this electrode between the depletion zone and the semi-insulating substrate.
A junction FET has as a gate the semiconductor of a type opposite that which forms the depletion zone. The FETs with a GaAs have as gate GaAs of type P.
The heterojunction FETs, which one skilled in the art knows concerns the use of two different semiconductors has a gate formed of the P type. The transistor works in charge depletion in the active N type layer doped at the upper level to 10.sup.16 at/cm.sup.3.
The frequencies obtained with these types of FET, although high are not yet sufficient, for the multiplicity of communications, especially by satellite. The overcrowding of the frequency bands results in unceasingly pushing the frequencies used back towards bands not yet used.
It is to comply with these requirements that a new field effect transistor has been perfected. Based on a heterojunction between materials of different mixtures, its operation relies on the existance of a type N channel whose conductance can be modulated by action on the polarization of an N type gate. This gate is a semiconductor having a wider inhibited band than that of the material supporting the channel, with the material being the purest possible N type with doping less than 10.sup.16 e/cm.sup.3. The cut-off frequency of such a transistor is about 30% higher than that of known MES-FETs.
This semiconductor device constitutes a progress in relation to the prior art and yet presents two types of limitation:
in the first place, the potential barrier between the GaAs layer and the Al.sub.x Ga.sub.1-x As layer may not be sufficiently high to prevent a considerable leakage of current due for example to the tunnel effect between the gate and the active layer when a positive voltage is applied to the gate,
in the second place, as a result of to the arrangement of the source and drain in relation to the gate region, the source-drain current crosses two zones formed from weakly doped GaAs, and is thus highly resistive. The access resistances equivalent to these two zones and called R.sub.s between source and gate and R.sub.d between gate and drain are high. Thus they limit the performance of the transistors at high frequency. The structures proposed allow nevertheless resistances R.sub.s and R.sub.d to be considerably reduced by reducing the dimensions of the resistive zones with a low doping level. But the resistances R.sub.s and R.sub.d thus obtained remain however higher than those met with in transistors of the prior art such as those called MES-FETs.
The invention is then completed by two improvements, of which:
the first consists in reducing the leakage current between the gate and the active layer to values comparable with those which are met with in field effect transistors of the prior art. This result is obtained by reducing the thickness of the Al.sub.x Ga.sub.1-x As layer to a few hundred angstrom and by inserting an insulating material between this Al.sub.x Ga.sub.1-x As layer and the gate metallization;
and the second consists in reducing the access resistances R.sub.s and R.sub.d to values comparable with those which are obtained in field effect transistors of the prior art. This result is obtained by bringing the source and drain regions into the vicinity of the electron accumulation zone which is in the active GaAs layer and adjacent the Al.sub.x Ga.sub.1-x As layer, thus suppressing the path of the source-drain current in the resistive zones with low doping level.